In certain electrical configurations, it is desirable to maintain a substantially constant output voltage. This may be the case, for example in a power supply, power stage, power conditioner, voltage source, or other similar circuit, which are all referred to collectively herein as a “voltage source.” A theoretical ideal voltage source has infinite input impedance, zero output impedance, infinite current capacity, and infinite slew rate so that it can theoretically provide a constant output voltage v (for direct current) or v(t) (for alternating current), regardless of the load current being drawn. For convenience, the actual output voltage of a voltage source, whether alternating current or direct current, may be referred to throughout this specification as Vout, and the ideal output voltage may be referred to as V0.
As a practical matter, a voltage source provides an imperfect time- or frequency-varying voltage. In the frequency domain, the output voltage may be described as Vout=V(s)=V0+Td(s)+K, where Td(s) represents a frequency-variant disturbance function and K represents a constant offset value. Both Td(s) and K may depend both on factors internal to the voltage source and external to the voltage source. Internal factors include for example leakage, internal noise, and offsets. External factors include for example timing, power demands including current transients, and feedback. Td(s) may also be dependent on noise in the circuit.